JP3054897B2 - Cleaning method for molten steel in tundish - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is intended to prevent molten steel contamination at the beginning of casting and to increase the speed of removing inclusions at the start of pouring molten steel from a ladle into a tundish in continuous casting of steel. The present invention relates to a method for cleaning internal molten steel.

[0002]

2. Description of the Related Art In continuous casting of steel, a tundish is located between a ladle and a mold and is one of the most important parts in operation and quality.

[0003] Its functions are to control the supply of molten steel into a mold, to store molten steel, and to separate and remove inclusions. In particular, the function of removing inclusions has become an extremely important function with the recent strictness of steel material quality.

However, when pouring molten steel from a ladle into a tundish, there is a problem of molten steel contamination due to air oxidation, and at present, the function of removing inclusions in the tundish is not sufficiently exhibited.

For this reason, various methods have been studied for the purpose of preventing molten steel contamination in a tundish and have been put to practical use.

For example, as described in JP-A-59-1055, air oxidation of injected molten steel is prevented by blowing an inert gas into a tundish covered with a refractory board lid. I have.

Further, Japanese Patent Application Laid-Open No. Sho 60-261,651 discloses a method of suppressing air oxidation by limiting the rate of rise of the molten metal level in a tundish when pouring molten steel from a ladle. In the publication, the ladle long nozzle which transfers molten steel from the ladle to the tundish
A method of efficiently removing inclusions in molten steel by adding a system alloy or a Ca-based flux has been proposed and has exerted a certain effect.

[0008]

However, with respect to the method of covering the tundish with a fireproof board lid and sealing with an inert gas, the tundish and the fireproof board themselves are deformed at a high temperature, so that the tundish is prevented to the extent that air oxidation can be prevented. The oxygen partial pressure in the dish cannot be reduced.

Also, since the bubbles entrained by the injection flow at the beginning of casting are very fine, the area of the reaction interface becomes large, and the air oxidation rate is much higher than that in the steady injection flow.

For this reason, the method of limiting the rate of rise of the molten metal level in the tundish in the early stage of casting has the effect of suppressing air oxidation to some extent, but does not provide sufficient pollution prevention measures.

Further, although the method of adding a Ca-based alloy or a Ca-based flux from a ladle long nozzle has the effect of increasing the removal rate of inclusions, the inclusions that have floated are again caught by the injection flow. The cleanliness of molten steel has not yet been secured.

In view of these problems, the present invention does not require a large-scale facility such as a refractory board cover, and reliably prevents the most severe molten steel contamination at the beginning of casting by a simple method. It is an object of the present invention to provide a method for cleaning molten steel in a tundish that also improves the removal function.

[0013]

SUMMARY OF THE INVENTION According to the present invention, in continuous casting of steel, molten steel is poured from a ladle into a tundish .
At the time, put the meltable heat insulator in the tundish,
There is a cleaning how the tundish molten steel, characterized in that to supply into the tundish at the beginning of molten steel injection, the addition of Ca.

[0014]

[Function] Generally, when pouring molten steel from a ladle into a tundish, the pouring flow involves a large amount of oxygen in the atmosphere until the ladle long nozzle is immersed below the surface of the molten metal.
A large number of inclusions are generated in the molten steel due to the air oxidation represented by the reaction of the formula (1).

[0015]

Embedded image 4 Al (in molten steel) + 3O ₂ (in air) = 2Al ₂ O ₃ (inclusions) (1)

Further, since oxygen is entrained in the molten steel as very fine bubbles, its oxidation rate is extremely faster than the oxidation rate during steady injection and the reaction rate with slag, and the maximum rate of molten steel contamination in the tundish. Cause.

Therefore, the present inventors have conducted various studies on a method for suppressing the air oxidation in the initial stage of casting and promoting the floating separation of the generated inclusions and the inclusions brought in from the ladle. As a result, it has been found that the problem can be solved by supplying a melting heat insulating agent and adding Ca.

FIG. 1 is a diagram for explaining the present invention in more detail. When the molten heat insulating agent 2 is put in the tundish 1 in advance and the molten steel 4 is poured from the ladle 3 into the tundish 1, the Ca-containing wire 6 is melted in the tundish 1 using the wire adding device 5. Add near the injection point. Ca is a component that is more active than Al and easily reacts with oxygen.

For this reason, the Ca component dissolved in the molten steel reacts with oxygen entrained at the time of injection according to the formula (2),
The formation of Al ₂ O ₃ -based inclusions caused by the reaction of the formula (1) is suppressed.

[0020]

Embedded image 2 Ca (in molten steel) + O ₂ (in air) = 2CaO (2)

Further, C formed by the reaction of the formula (2)
aO and Al ₂ O ₃ inclusions brought in from the ladle (3)
Reacts according to the formula, producing low melting point inclusions.

[0022]

Embedded image mCaO + nAl ₂ O ₃ (inclusion) = (CaO) _n. (Al ₂ O ₃ ) _n (3)

Since the coalescence of the low-melting inclusions is completed in a very short time, the addition of Ca has the effect of accelerating the coalescence of the inclusions and increasing the flotation speed.

However, when the inclusions of the heat insulating agent have a low ability to absorb inclusions, the inclusions that have floated stay near the surface of the molten metal and re-engage, so that the ultimate value of the concentration of the inclusions is not improved.

On the other hand, when the meltable heat insulating agent is supplied, the heat insulating agent becomes a liquid phase and the inclusion absorbing ability becomes high, so that the ultimate value of the inclusion concentration can be reduced and the cleanliness can be improved.

FIG. 2 shows the results of the effect of Ca and the effect of the heat insulator determined by crucible experiments.

As can be seen from the figure, the addition of Ca has the effect of accelerating the reduction rate of the total oxygen content (inclusion concentration), and the use of the fusible heat insulating agent has the effect of reducing the ultimate value of the total oxygen content (inclusion concentration). Have.

Therefore, by using these together in the tundish, it is possible to reliably prevent the molten steel from being contaminated by air entrainment at the initial stage of injection, and to improve the function of removing inclusions in the tundish.

Ca may be added in powder form or in a state of being surrounded by a wire. The appropriate amount of Ca to be added cannot be specified unconditionally, and it is necessary to adjust according to the amount of air involved in the injection flow and the amount of inclusions brought in from the ladle, but the outline of the determination method is as follows. The change over time in the total oxygen amount near the molten steel injection point may be evaluated, and the Ca amount corresponding thereto may be added in consideration of the yield.

The wire is made of Ca from the viewpoint of improving the Ca yield.
It is effective to use an Al wire, a Ca—Si wire, or the like, and the selection of the wire type uses a material that does not cause a problem in composition and needs to be changed according to the steel type.

It is effective to use a low-viscosity fusible insulating material which is as low as possible in viscosity and excellent in the ability to absorb inclusions. However, a low-viscosity fusible insulating material violently dissolves a refractory in a tundish. It is necessary to reduce the viscosity within a range where does not cause a problem.

Further, when the heat insulator contains a large amount of lower oxides such as SiO _2, it reacts with Al in the molten steel to generate inclusions, so that the steel is contaminated by the heat insulator.

For this reason, it is effective to reduce the content of the lower oxide in the heat insulating agent as much as possible.

Here, CaO + A is used as the melting heat insulating agent.
_{l 2 O 3, CaO + Al} 2 O 3 + things such as MgO is convenient.

Regarding the method of supplying the heat retaining agent, a similar effect can be obtained by a method in which Ca is added simultaneously with the start of molten steel injection, in addition to the method of placing it before the start of molten steel injection as described above.

[0036]

The present invention will be described below with reference to examples and comparative examples.

[0037]

Example 1 A fusible warming agent having a CaO content of 50%, an Al ₂ O ₃ content of 50% and a melting point of 1400 ° C. was previously placed in a 50 t capacity tundish.

Component C: 50 ppm, Si: 0.015
%, Mn: 0.25%, P: 0.02%, S: 0.01
%, Al: 0.035%, temperature of 1550 ° C. (in the tundish) molten steel 250t is started to be poured from the ladle into the tundish, and Ca: 50%, Al: 20%,
A Fe: 30% Ca-Al wire (7 mmφ) was added near the tundish injection point.

The addition rate of the wire is 2 kg / min for 50 t of molten steel until the tundish is filled with molten steel and air entrainment at the beginning of injection is completed, and 700 g / min for subsequent molten steel. min.

At this time, the total amount of oxygen in the molten steel on the discharge side of the tundish could always be stably maintained at 20 ppm from the beginning of the injection, and the contamination of the molten steel could be reliably prevented.

[0041]

COMPARATIVE EXAMPLE 1 A solid heat insulating agent having a MgO content of 100% and a melting point of 2800 ° C. was previously placed in a 50-ton tundish.

Component C: 50 ppm, Si: 0.015
%, Mn: 0.25%, P: 0.02%, S: 0.01
%, Al: 0.035%, 250 t of molten steel at a temperature of 1550 ° C. (within the tundish) was poured from a ladle into the tundish, and the Ca-containing wire was not added.

During the 15 minutes from the start of casting, the total oxygen content in the molten steel gradually decreased from 140 ppm to 60 ppm, but thereafter, the decrease in the total oxygen content was stagnant and the final reached value was 60 p.m.
pm.

For this reason, no improvement was observed in the function of preventing molten steel contamination and the function of removing inclusions at the initial stage of the injection, and surface defects occurred in the product after rolling.

[0045]

Comparative Example 2 A solid warming agent having a MgO content of 100% and a melting point of 2800 ° C. was placed in a 50-ton tundish.

Component C: 50 ppm, Si: 0.015
%, Mn: 0.25%, P: 0.02%, S: 0.01
%, Al: 0.035%, temperature 1550 ° C (in the tundish) 250 t of molten steel was started to be poured from the ladle into the tundish, and Ca: 50%, Al: 20%,
A Fe: 30% Ca-Al wire (7 mmφ) was added near the tundish injection point.

The wire was added at a rate of 2 kg / min for 50 t of molten steel until the tundish was filled with molten steel and the air entrainment at the beginning of injection was completed, and 700 g / min for subsequent molten steel. min.

At this time, the total oxygen content in the molten steel on the discharge side of the tundish was constant at 60 ppm from the initial injection to the end of the 250 t injection, and although the function of preventing molten steel contamination at the initial casting was obtained,
The inclusion removal function did not improve. For this reason, surface defects occurred in the product after rolling.

[0049]

As described above, according to the method for cleaning molten steel in a tundish of the present invention, large-scale facilities are not required, and the most severe molten steel contamination in the early stage of casting can be reliably prevented.
Furthermore, since the inclusion removal function in the tundish can be improved, the quality of the slab is extremely improved.

[Brief description of the drawings]

FIG. 1 is a view for explaining a method for cleaning molten steel in a tundish of the present invention.

FIG. 2 is a graph showing the effects of Ca addition and the use of a meltable heat insulating agent, which were evaluated by a crucible test.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Tundish 2 Melt heat insulator 3 Ladle 4 Molten steel 5 Wire addition device 6 Ca containing wire

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-61-1457 (JP, A) JP-A-3-243252 (JP, A) JP-A-2-270907 (JP, A) JP-A 61-157 38749 (JP, A) JP-A-59-1055 (JP, A) JP-A-52-35126 (JP, A) JP-A-63-149057 (JP, A) JP-A-54-86433 (JP, A) JP-A-60-261652 (JP, A) JP-A-61-276756 (JP, A) JP-A-1-99761 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/11 B22D 11/108 B22D 11/111

Claims (1)

(57) [Claims] 1. In a continuous casting of steel, when a molten steel is poured from a ladle into a tundish, a molten heat insulating agent is added to the tundish.
Or in a tandem at the beginning of molten steel injection.
A method for cleaning molten steel in a tundish , comprising supplying Ca into a dish and adding Ca.